Influenza A virus comprises eight segmented RNA fragments, and 14 viral proteins can be encoded by utilizing the RNA fragments. The completion of replication and transcription of the viral genome need a functional unit, RNP complex. Virus, when infecting host cells, is faced with 2 barriers and 4 shuttles.
The first barrier is cytoplasmic membrane, which is passed through by the virus when it enters a cell, at this time the virus binds to a sialic acid receptor on the surface of the cell using hemagglutinin protein HA, and invades into the inside of the host cell via endocytosis.
Matrix protein M1 then releases vRNP complex into the cytoplasm, since genomic replication and transcription need to occur in the nucleus, at this time, the vRNP complex is faced with the second barrier, the nuclear membrane. VRNP binds to the nuclear transport receptor protein importin-α using atypical bidirectional NLS at N-terminal of the NP protein, so as to pass through the nuclear pore complex and enter the inside of the nucleus to initiate replication and transcription.
The mRNA generated by transcription is translated in the cytoplasm, and the newly synthesized viral polymerase components, in turn, use their own NLS to enter the nucleus, respectively and reassemble into RNP complex.
After replication of the viral genome is completed, the RNP complex will utilize the NP protein, together with the M1 protein and the NEP protein, to form a large complex, which is transported to the cytoplasm by the cytoplasmic transport protein CRM1 protein, such that another barrier shuttle is completed, and then complete virus particles are assembled.
The NP protein plays an important role in the process of two nuclear membranes shuttles.